Science of Photography - Diffraction Limit

Diffraction Limit

Since light propagates as waves, the patterns it produces on the film are subject to the wave phenomenon known as diffraction, which limits the image resolution to features on the order of several times the wavelength of light. Diffraction is the main effect limiting the sharpness of optical images from lenses that are stopped down to small apertures (high f-numbers), while aberrations are the limiting effect at large apertures (low f-numbers). Since diffraction cannot be eliminated, the best possible lens for a given operating condition (aperture setting) is one that produces an image whose quality is limited only by diffraction. Such a lens is said to be diffraction limited.

The diffraction-limited optical spot size on the CCD or film is proportional to the f-number (about equal to the f-number times the wavelength of light, which is near 0.0005 mm), making the overall detail in a photograph proportional to the size of the film, or CCD divided by the f-number. For a 35 mm camera with f/11, this limit corresponds to about 6,000 resolution elements across the width of the film (36 mm / (11 * 0.0005 mm) = 6,500.

In other words, for two distant point to appear as separate objects on the film or sensor, the distance from each to the two sides of the open lens aperture must differ by at least a half wavelength; otherwise they will be both in phase at the same point on the film or CCD and won't be distinguished from each other.